The tumoricidal activity of C3H mouse alveolar macrophages (AM) was completely abolished by 100 Gy gamma-irradiation, whereas the activities of AM of C57Black/6 (B6) and Balb/c mice were not significantly affected. The viability of AM, determined by both vital staining with crystal violet and by trypan blue exclusion, was decreased markedly by gamma-irradiation only in C3H mouse AM. AM of B6, Balb/c mice and four strains of rat were much more radioresistant than C3H mouse AM with respect to cell viability. On the other hand the effect of irradiation on the colony-forming capacity of AM was essentially the same in three strains of mouse. Morphological examination of irradiated C3H mouse AM and the DNA migration pattern in agarose gel electrophoresis indicated that gamma-irradiation induced high levels of apoptosis in C3H mouse AM. These results suggest that the apoptotic cell death induced by gamma-irradiation at doses that do not affect AM of the other strains of mouse and rat explains the radiosensitivity of C3H mouse AM with respect to cell viability and tumoricidal activity.